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Linux containers and Docker Elvin Sindrilaru IT Storage Group - CERN GridKa School 2016 Outline • Understanding Linux containers • Linux namespaces • Linux cgroups • Docker containers • Containers orchestration • Security considerations • Benefits 30/08/2016 Linux containers and Docker 3 What is a container? "Cmglee Container City 2" by Cmglee - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons 30/08/2016 Linux containers and Docker 4 Linux containers • Based on two technologies • Linux namespaces • Linux control groups (cgroups) 30/08/2016 Linux containers and Docker 5 Linux namespaces (1) • The purpose of a namespace is to wrap a particular global system resource in an abstraction that makes it appear to the process within the namespace that they have their own isolated instance of the global resource. 30/08/2016 Linux containers and Docker 6 Linux namespaces (2) • Currently there are 6 namespaces implemented in the Linux Kernel: • Mount namespaces – isolate the set of file-system mount points seen by a group of processes (Linux 2.6.19) • UTS namespaces – isolate two system identifiers – nodename and domainname. (UNIX Time-sharing System) • IPC namespaces – isolate inter-process communication resources e.g. POSIX message queues 30/08/2016 Linux containers and Docker 7 Linux namespaces (3) • Network namespaces – provides isolation of system resources associated with networking. Each network namespace has its own network devices, IP addresses, port numbers etc. • PID namespaces – isolate process ID number space. Processes in different PID namespaces can have the same PID number. • User namespace – isolates the process user and group ID number spaces. A process’s UID and GID can be different inside and outside a user namespace i.e a process can have full root privileges inside a user namespace, but is unprivileged for operations outside the namespace. (Linux 3.8) 30/08/2016 Linux containers and Docker 8 Linux cgroups (1) • Cgroups allow allocating resources to user-defined groups of processes running on the system • Cgroup subsystems (resources controllers) = kernel modules aware of cgroups which allocate varying levels of system resources to cgroups • Everything is exposed through a virtual filesystem: • /cgroups, /sys/fs/cgroup … - mountpoint may vary • Currently up to 10 subsystems: • blkio – set limits on input/output access to/from block devices such as physical drives • cpuset – assign individual CPUs and memory nodes to tasks in a cgroup • memory – set limits on memory used by tasks in a cgroup • etc … 30/08/2016 Linux containers and Docker 9 Linux cgroups (2) • libcgroup package provides command line utilities for manipulating cgroups. • lssubsys – list available subsystems • lscgroup – list defined cgroups • cgget – get parameters of cgroup • cset – set parameters of cgroup • cgexec – start a process in a particular cgroup • cgclassify – move running task to one or more cgroups 30/08/2016 Linux containers and Docker 10 Linux containers a.k.a LXC • Containers • tool for lightweight virtualization • provides a group of processes the illusion that they are the only processes on the system • Advantages in comparison to traditional VM: • Fast to deploy - seconds • Small memory footprint - MBs • Complete isolation without a hypervisor Namespaces + Cgroups => Linux containers 30/08/2016 Linux containers and Docker 11 Linux containers – CLI • lxc package contains tools to manipulate containers • lxc-create • Setup a container (rootfs and configuration) • lxc-start • Boot a container • lxc-console • Attach a console if started in background • lxc-attach • Start a process inside a container • lxc-stop • Shutdown a container • lxc-destroy • Destroy the container created with lxc-create 30/08/2016 Linux containers and Docker 12 Docker containers • “A container is a basic tool, consisting of any device creating a partially or fully enclosed space that can be used to contain, store and transport objects or materials.” http://en.wikipedia.org/wiki/Container • “Open-source project to easily create light-weight, portable, self-sufficient containers from any application” https://www.docker.com/whatisdocker/ • Docker motto: “Build, Ship and Run Any App, Anywhere” 30/08/2016 Linux containers and Docker 13 Docker interaction • Client-server model • Docker daemon • Process that manages the containers • Creates files systems, assigns IP addresses, routes packages, manages processes => needs root privileges • RESTful API • Docker client • Same binary as the daemon • Makes GET and POST request to the daemon 30/08/2016 Linux containers and Docker 14 Docker client-server interaction • The client can run on the same host or on a different one from the daemon 30/08/2016 Linux containers and Docker 15 VMs vs. Docker containers • VMs are fully virtualized • Containers are optimized for single applications, but can also run a full system 30/08/2016 Linux containers and Docker 16 Docker filesystem • Typical Linux system needs two filesystems: • Boot file system (bootfs) • Root file system (rootfs) - /dev, /etc, /bin, /lib … © Docker Inc. • Docker can use Another Unionfs (AUFS) which is copy-on-write • AUFS • Helps sharing common portions of the fs among containers • Layers are read-only and the merger of these layers is visible to the processes • Any changes go into the rd/wr layer 30/08/2016 Linux containers and Docker 17 Docker Images • Image • Never changes • Stack of read-only fs layers • Changes go in the topmost © Docker Inc. writable layer created when the container starts • Changes are discarded by default when the container is destroyed • Where to get Docker Images from? • https://registry.hub.docker.com/ • Similar to what GitHub is for Git - think “git repository for images” • Use your own private registry e.g. pull the docker registry image and run it in a container 30/08/2016 Linux containers and Docker 18 Docker Containers • Container • Read-write layer • Information about Parent Image (RO layers) • Unique id + network configuration + resource limits • Containers have state: running / exited • Exited container • preserves file system state • does NOT preserve memory state • Containers can be promoted to an Image by using “docker commit” Takes a snapshot of the whole filesystem (RW+RO) 30/08/2016 Linux containers and Docker 19 Docker paradigm shift • Motto: “write-once-run-anywhere” • Developers: • Concentrate on building applications • Run them inside containers • Avoid the all too common: “But it works fine on my machine …” • Sysadmins/operations/DevOps: • Keep containers running in production • No more “dependency hell” … almost … at least not traditional ones ⇒ Clean separation of roles ⇒ Single underlying tool which (hopefully) simplifies: ⇒ code management ⇒ deployment process 30/08/2016 Linux containers and Docker 20 Docker workflow automation • Dockerfile • Repeatable method to build Docker images – makefile equivalent • DSL(Domain Specific Language) representing instructions on setting up an image • Used together with the context by the “docker build” command to create a new image # Use the fedora base image FROM fedora:20 MAINTAINER Elvin Sindrilaru, [email protected], CERN 2016 # Add a file from the host to the container ADD testfile.dat /tmp/testfile # Install some packages RUN yum -y --nogpg install screen emacs # Command executed when container started CMD /bin/bash 30/08/2016 Linux containers and Docker 21 Containers orchestration • Orchestration describes the automated arrangement, coordination and management of complex systems, middleware and services. • Library dependencies “sort of” become container dependencies 30/08/2016 Linux containers and Docker 22 Container data management • Docker volume • Directory separated from the container’s root filesystem • Managed by the docker daemon and can be shared among containers • Changes to the volume are not captured by the image • Used to mount a directory of the host system inside the container • Data-only containers • Expose a volume to other data-accessing containers • Prevents volumes from being destroyed if containers stop or crash 30/08/2016 Linux containers and Docker 23 Port binding and linking containers • Bind container ports to host ports using the “-p” flag • Not all containers need to bind internal ports to host ports • E.g. only front-end applications need to connect to backend services • Linking within the same host • Profit from the unified view that the docker daemon has over all running containers • Use the --link option: docker run --link CONTAINER_ID:ALIAS … • Effectively alters the /etc/hosts file • Cross host linking • Requires a distributed, consistent discovery service • Needs a distributed key-value store to keep info about running containers e.g. etcd • Application must be aware of the discovery service 30/08/2016 Linux containers and Docker 24 Orchestration tools/frameworks • Docker Machine, Compose and Swarm • Kubernetes • https://github.com/GoogleCloudPlatform/kubernetes • Shipyard • https://github.com/shipyard/shipyard • LXC/LXD/CGManage • https://linuxcontainers.org/ 30/08/2016 Linux containers and Docker 25 Security considerations • Docker containers are started with a reduced capability set which restricts: • Mount/unmount devices • Managing raw sockets • Some fs operations • Fine-grained control of capabilities using the docker -- cap-add --cap-drop options • End-goal is to run even the Docker daemon as a non-root user and delegate operations to dedicated subprocesses • Keep the host Kernel updated with the latest security patches 30/08/2016 Linux containers and Docker 26 Docker – Benefits • Portable deployment across machines – overcomes machine specific configuration issues • Application-centric – optimized for deployment of applications and not machines • Automatic build – can use any configuration management system (puppet, chef, ansible etc.) to automate the build of containers • Versioning - git like capabilities to track container versions • Component reuse – any container can become a base image • Sharing – use the public registry to distribute container images, just like a git repository 30/08/2016 Linux containers and Docker 27 .
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